Molecular Mechanisms of Dendrite Stability

REVIEWS

Molecular mechanisms of dendrite stability

Anthony J. Koleske Abstract | In the developing brain, dendrite branches and dendritic spines form and turn over dynamically. By contrast, most dendrite arbors and dendritic spines in the adult brain are stable for months, years and possibly even decades. Emerging evidence reveals that dendritic spine and dendrite arbor stability have crucial roles in the correct functioning of the adult brain and that loss of stability is associated with psychiatric disorders and neurodegenerative diseases. Recent findings have provided insights into the molecular mechanisms that underlie long-term dendrite stabilization, how these mechanisms differ from those used to mediate structural plasticity and how they are disrupted in disease.

The proper formation and long-term maintenance of as it affords mature neurons the ability to fine-tune spine- neuronal connectivity is crucial for correct function- based synaptic connections, while retaining overall long- ing of the brain. The size and shape of a neuron’s den- term dendrite arbor field integrity and integration within drite arbor determine the number and distribution of networks. Furthermore, cytoskeletal stability is crucial receptive synaptic contacts it can make with afferents. for maintaining long-lasting synaptic changes such as During development, dendrites undergo continual long-term potentiation (LTP). Examining the distinct dynamic changes in shape to facilitate proper wiring, mechanisms that mediate spine and dendrite stability is synapse formation and establishment of neural circuits. the major focus of this Review. Dendrite arbors are highly dynamic during develop- By adulthood, the dynamic behaviour of spines is ment, extending and retracting branches as they mature, greatly reduced. Transcranial two-photon imaging indi- and only a subset of nascent dendrite branches become cates that a large fraction of dendritic spines in the adult stabilized1–4 (FIG. 1). During this early wiring period, rodent cortex are stable for extended time periods of synapse and dendrite arbor stabilization are intimately several months and possibly years15–18,27 (FIG. 1). Together, connected. For example, synapse formation on a nascent these findings suggest a scenario in which most dendritic dendrite branch promotes its stabilization, whereas the spines and dendrite arbors become stabilized for long loss or reduction of synaptic inputs destabilizes target periods of an organism’s lifetime, perhaps even for dendrites4–13. decades in humans. The structural plasticity of dendrites decreases Losses of dendritic spine and dendrite arbor stability greatly as circuits mature (FIG. 1). Most dendrite branches in humans are major contributing factors to the pathol- become stabilized first while individual dendritic spines ogy of psychiatric illnesses such as schizophrenia and continue to form, change shape and turn over as circuits major depressive disorder (MDD), neurodegenerative refine14–18. During this period, the formation and prun- diseases, such as Alzheimer’s disease, and damage from ing of spines is particularly sensitive to experience and stroke. Importantly, different patterns of dendritic spine Departments of Molecular activity patterns16,18–20. This is followed by a period of and dendrite branch loss are observed in different psy- Biophysics and Biochemistry and Neurobiology, extensive synapse and dendritic spine pruning, which chiatric and neurodegenerative disorders (reviewed in Yale University, can can last throughout adolescence and early adulthood REF. 28), suggesting that spine and branch stabilization 333 Cedar Street SHMC‑E31, in some human brain regions17,20–26. In stark contrast to mechanisms are differentially disrupted in different New Haven, Connecticut early development, in which stabilization of dendrite disease pathologies. The altered synaptic connectivity 06520–8024, USA. branches depends critically on synapse formation, den- resulting from dendrite arbor and dendritic spine desta- e‑mail: anthony.koleske@ yale.edu dritic spine and dendrite branch stability become mecha- bilization is thought to contribute to the impaired per- doi:10.1038/nrn3486 nistically uncoupled during this late refinement period. ception, cognition, memory, mood and decision-making Published online 10 July 2013 Such uncoupling is crucial for long-term circuit stability, that characterize these pathological conditions.

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a b c E15–P21 P21–P60 P60 onwards

Few spines

Spines dynamic Spines dynamic Spines (mostly) Axon stabilize

Branches dynamic Branches stabilize Branches stabilize

Figure 1 | Dendrite branch and dendritic spine dynamics change during development. Naturea | During Reviews early |development Neuroscience in mice (embryonic day 15 (E15) to postnatal day 21 (P21)), dendritic branches are highly dynamic, extending new branches (green) and retracting some existing branches (red). Failure to form productive synaptic contacts (inset, red dendrite segment) results in fewer spines and dendrite branch retraction; more stable branches (inset, green dendrite segment) contain a mix of stable spines, new spines and destabilizing spines. b | As animals enter and transit adolescence (P21–P60), some dendrite branches stabilize, while a fraction of dendritic spines remain dynamic, with a net loss of spines. c | As animals enter adulthood, dendritic spine dynamics slow and most of the spines remain stable.

A growing number of recent studies have begun to Direct measurements using sophisticated live micros- dissect the mechanisms that mediate long-term den- copy techniques reveal that actin exists in distinct dritic spine and dendrite branch stability. Here, I pro- pools that differ in their movement and stability vide an up‑to‑date review of the molecules (TABLE 1) and within the spine36,43,44. Photoactivation studies indicate cellular and molecular mechanisms that differentially that actin at the spine tip and periphery, also known as regulate dendritic spine versus dendrite branch stabil- the spinoskeleton ‘shell’, is dynamic, cycling between ity and highlight how these mechanisms are targeted by monomeric globular and polymeric (filamentous) pathology. forms that turn over with half-lives of tens of seconds. By contrast, actin in the centre and base of the spine, Determinants of dendrite and spine stability also known as the spinoskeleton ‘core’, turns over in Dendritic spines are supported by an F‑actin framework. At tens of minutes43. Consistent with this model, when most excitatory synapses, presynaptic boutons synapse onto single photoactivated actin molecules were tracked in small dendritic spines that emerge from the dendritic shaft. spines, half remained largely stationary, as would be Spines have diverse shapes, including thin, short and stubby, expected of actin associated with a more stable actin and mushroom-shaped, with a club-like head and thinner core, whereas one-third underwent slow retrograde neck15,29–32. Dendritic spines are enriched in filamentous (F) movement, which is consistent with a more dynamic actin, which provides the spine shape, organizes the postsyn- pool of actin associated with the shell44. It has recently aptic signalling machinery, drives changes in spine structure been noted that distinct actin-binding proteins (ABPs) and maintains spine stability33–37. The spine cytoskeleton localize to discrete regions within the spinoskeleton38. (referred to here as the ‘spinoskeleton’)38 is composed of a For example, cortactin, which both stimulates actin mix of linear and branched actin networks, which extend polymerization and stabilizes branched actin net- from the base of the spine to the postsynaptic density works, localizes to the spinoskeleton core45, whereas (PSD)31,39. This actin network fills the spine — it is closely cofilin, an F‑actin-severing protein that is associated apposed to the spine membrane, and it retains this associa- with dynamic F‑actin turnover, localizes to the shell46 tion during activity-driven remodelling of spine structure31 (FIG. 2). Together, these studies suggest that the periph- (FIG. 2). Changes in the amount and structure of F‑actin also ery of spine structure — near the outer membrane mediate long-lasting alterations in spine size and synaptic — is likely to be capable of greater morphological efficacy. For example, repetitive firing of synapses, such as change, whereas the central core remains more sta- that occurring during high-frequency synaptic stimulation ble. This is consistent with the observation that spines to induce LTP, promotes actin polymerization within the can be highly motile and could reflect a need, during spine, causing the spine to enlarge40,41. Conversely, treat- synaptogenesis, to keep the overall position of new ment that weakens synaptic efficacy, such as low-frequency spines constant while the more dynamic outer portion stimulation that results in long-term depression, causes actin searches for suitable synaptic contacts or adjusts in size loss and dendritic spine shrinkage40,42. in response to new activity patterns.

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Table 1 | Molecules influencing dendritic spine and dendrite arbor stability actin filaments and microtubules that make up their structures turn over in matters of minutes to hours; this Key molecules Refs is the case in both the developing and mature nervous Stabilizes spines RAC1 66 systems. Over 80% of F‑actin in spines turns over every WAVE1 69,70 minute36,43,44, whereas 75% of the microtubules in den- 52,53 Cortactin 76,84,89 drites turn over within tens of minutes . It is important to note that the remaining proportion of F‑actin β‑adducin 96,97 and microtubules undergo depolymerization-mediated EPHB–FAK 98 turnover in dendrites but at a slower rate — on the order FYN 100 of tens of minutes for spine F‑actin and a probably a few hours for dendrite shaft microtubules, as has been p130CAS 101 measured for axon microtubules52,53. MARCKS 78,203 Given the high turnover of molecules that confer p140CAP 85 structural integrity, how is structural stability and Stabilizes dendrites MAP1A 60,61 functional integrity achieved? The secret appears to lie in the precise regulation of the molecules that con- MAP2 62,63 trol dendritic cytoskeletal dynamics. Dendritic spines p190RHOGAP 77,114 and shafts are replete with proteins that promote the NDR1 and NDR2 138 formation of actin filaments and microtubules, respec- Cypin 142,143 tively, and organize and stabilize these cytoskeletal structures (FIG. 2). In addition to shaping the cytoskel- BDNF–TRKB 108,140,141 Stabilizes both spines eton, these proteins ensure that only a fraction of the and dendrites Integrin α3β1 101,114,115 cytoskeleton undergoes remodelling at any given time ARG 26,77,114 and that existing actin filaments and microtubule net- EB3 85,204 works can both maintain dendritic structure and serve as scaffolds for their own replenishment. For exam- Destabilizes both RHOA–ROCK 68,71,72,147–151 ple, the actin-related protein 2/3 (ARP2/3) complex, spines and dendrites Amyloid-β-derived diffusible ligands 175–178,181 which localizes to dendritic spines54, has been shown Tau and tau kinases 179,181 to nucleate new actin branches from the side of an existing actin filament in vitro55. Selective inactivation GATA1 185 of the ARP2/3 complex in excitatory neurons leads to Corticosteroids 205–208 significant spine and synapse loss in cortical and hip- Increased NMDAR activation 84,89,198–202 pocampal neurons, further supporting a crucial role for BDNF, brain-derived neurotrophic factor; EB3, end-binding protein 3; FAK, focal adhesion kinase; actin replenishment in the maintenance of spine and GATA1, GATA-binding factor 1; MAP, microtubule-associated protein; MARCKS, myristoylated synapse structural integrity56. alanine-rich C‑kinase substrate; NDR, nuclear Dbf2‑related kinase; NMDAR, NMDA receptor; p130CAS, p130 Crk-associated substrate; p140CAP, p130 Cas-associated protein; p190RHOGAP, Dendritic microtubules also contain high concen- 190 kD RHO GTPase-activating protein A; ROCK, RHO-associated protein kinase. trations of microtubule-associated proteins (MAPs), including MAP1A and MAP2 (FIG. 2), both of which can promote microtubule polymerization and sta- Microtubule arrays in dendrite branches support morpho‑ bilize existing microtubules57–59. Upregulation of logical structural and membrane trafficking. In the mature MAP1A and MAP2 expression strongly correlates nervous system, the cytoskeleton within dendrite branches with dendrite stabilization in cultured neurons, and consists of a packed network of microtubules, which pro- both proteins are required for activity-dependent vides structural stability, anchors organelles and serves as a enlargement and stabilization of developing dendrite highway for the transport of cargoes, including all manner arbors60,61. Importantly, knockout of these proteins in of dendritic building materials and organelles47. In con- mice disrupts microtubule spacing within dendrites trast to microtubules in axons, which are polarized (with and leads to significant reductions in dendrite arbor the plus ends facing the end of the axon), microtubules size62,63. In addition to their effects on microtubule within dendrites are of mixed polarity, which may facili- spacing and stability, MAPs may also protect dentate bidirectional trafficking of cargoes by microtubule- dritic microtubules from the activities of endogenous based motors48,49. That said, polarized microtubule arrays microtubule-severing enzymes, such as katanin, which have been observed in large-diameter apical dendrites50, could otherwise destabilize dendrite arbors by trigger- possibly reflecting the bias of transport of cargoes away ing microtubule instability64. Covalent modifications of from soma out to their elaborate dendrite arbors. Serial dendritic microtubules that are associated with micro- electron micrographic reconstructions reveal that ordered tubule stability include detyrosination of the α‑tubulin dendritic microtubules lie regularly spaced, parallel to each subunit, which stabilizes the microtubules by mak- other and to the dendritic shaft51 (FIG. 2). ing them poor substrates for the kinesin‑13 family of microtubule depolymerases65. In summary, structural A dynamic cytoskeleton, but stable branches and spines. integrity of spines and dendrites is achieved by a care- Although the gross structure of dendritic spines and ful balance between stabilization and destabilization branches is stable for months or years, the individual mechanisms.

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AMPAR NMDAR

Shell

Spine

Core

Dendrite branches

Golgi outpost Microtubule nucleation

ARP2/3 complex MAP1A Cortactin Microtubule Coﬁlin Microtubule monomers Cypin Vesicle F-actin Microtubule-based MAP2 motors

Figure 2 | Cytoskeletal structure in dendrites. The cytoskeletal ultrastructure in a small region of a dendrite (boxed area on the neuron). Dendritic shafts contain large parallel bundles of microtubules bridged by microtubule-associated protein 1A (MAP1A) and MAP2. The microtubule array near a dendritic branch site servesNature as a docking Reviews site | Neuroscience for a Golgi outpost, which services the trafficking of vesicles (cargo) to and from the more terminal regions of dendritic membrane and serves as a site for microtubule nucleation. Cypin helps to promote the assembly and stability of new microtubules. The dendritic spine is supported by network of actin filaments. The more stable central core contains the actin-related protein 2/3 (ARP2/3) complex and the actin nucleation-promoting factor and actin branch stabilizer cortactin. F‑actin filaments are less densely packed and more dynamic in the spine shell, which is enriched in the actin-severing protein cofilin. AMPAR, AMPA receptor; NMDAR, NMDA receptor.

Molecular mechanisms of spine maintenance network stability emerges, possibly to help stabilize spines The long-term stability of dendrite and spine structure for the long-term. Importantly, all of these pathways con- in the mature nervous system thus intimately depends verge on a single target, the spine actin cytoskeleton, and on the actin control machinery, which is regulated by thus there is significant crosstalk between these mecha- a wide range of extracellular molecules that act on cell nisms. The roles of these molecules in regulating actin surface adhesion receptors, including EPH receptors, cytoskeletal dynamics in the spine are discussed in more immunoglobulin superfamily receptors, cadherins and detail below. integrins (FIG. 3). Upon engagement with their extracellular binding partners, these receptors influence a wide RHO‑family GTPases exert differential effects on spine range of downstream signalling molecules. These include stability. The RHO‑family GTPases RHOA, RAC1 and cytoplasmic non-receptor tyrosine kinases of the focal cell division control protein 42 homologue (CDC42) are adhesion kinase (FAK), SRC and ABL families that mod- central regulators of cytoskeletal dynamics in neurons66. ulate the activities of RHO‑family GTPases and other All of these GTPases are active in the spine, where they downstream cytoskeletal stabilizing proteins, and serve have been shown to mediate spine formation in devel- to control spinoskeletal structure and ensure long-term oping neurons and activity-based changes in spine size67. spine stability (FIG. 3). Some of these molecules operate Beyond this initial period of spinogenesis, these GTPases throughout neuronal development and into adulthood to continue to influence spine shape and stability, even into control actin dynamics in spines. In many cases, changes adulthood. For example, the introduction of dominant- in the expression and/or activity of these proteins closely negative RAC1 into hippocampal neurons leads to pro- follow changes in synaptic activity. Other spine signalling gressive spine loss, reflecting a requirement for RAC1 components become engaged only late in development as in long-term spine stability68. Among its downstream

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Ca2+

Ephrin B BDNF Extracellular EPHB2 p130CAS–CRK–DOCK1 matrix ? Integrin α3β1 TRKB FAK RAC1 RAC1 SRC WRC WAVE1 PAK1

LIMK1

MARCKS

ARP2/3 complex ARG Cortactin

Coﬁlin F-actin

Figure 3 | Cytoskeletal signalling pathways that stabilize spines. Several cytoskeletal signalling pathways mediate dendritic spine stabilization. On the left, presynaptic ephrin B binding to its receptorNature EPHB2 Reviews triggers |a Neuroscience focal adhesion kinase (FAK)–SRC kinase relay that activates the p130 Crk-associated substrate (p130CAS)–adaptor molecule CRK– dedicator of cytokinesis protein 1 (DOCK1) complex, which in turn activates the GTPase RAC1. RAC1 releases the WAVE regulatory complex (WRC) from WAVE1, allowing it to activate the actin-related protein 2/3 (ARP2/3) complex to nucleate a new actin branch. In response to an unknown signal, integrin α3β1 activates tyrosine protein kinase ARG and promotes its phosphorylation of and binding to cortactin. Cortactin can activate actin branch nucleation through the ARP2/3 complex and can stabilize the branches that are formed. ARG binding can also stabilize actin filaments and promote cortactin recruitment to actin filaments. On the right, presynaptic depolarization triggers Ca2+-dependent release of brain-derived neurotrophic factor (BDNF), which binds to the receptor tyrosine kinase TRKB to activate RAC1. RAC1 acts through a serine/threonine-protein kinase PAK1–LIM domain kinase 1 (LIMK1) cascade to phosphorylate and inhibit cofilin and block its severing of actin filaments. Myristoylated alanine-rich C kinase substrate (MARCKS) tethers actin filaments to the membrane to stabilize F‑actin in spines.

targets, RAC1 activates WAVE (also known as WASF) stimulates LIM kinase to phosphorylate cofilin and inhibit family proteins to stimulate the ARP2/3 complex, which its actin severing activity74,75, but this might be expected to (as explained above) is associated with F‑actin in the promote spine stability rather than reduce it (see below). spinoskeleton core and acts to nucleate new actin fila- Studies of the effects of synaptic activity on cytoskel- ment branches from existing actin filaments41,66,67 (FIG. 3). etal stability have revealed a more complex relationship WAVE1 also localizes to dendritic spines and knockdown between RHOA signalling and spine stability. Local or knockout of Wave1 causes reductions in dendritic spine uncaging of glutamate to stimulate individual spines density69,70. These observations strongly suggest that caused increased RHOA activity in the spine, which RAC1 acts through WAVE1 and the ARP2/3 complex mediated activity-based spine enlargement67. By con- to refresh the spinoskeleton core and therefore supports trast, blocking AMPA receptors in cultured hippocampal long-term spine stability. neurons with NBQX for several days caused increased In contrast to RAC1, activated RHO mutants or RHOA activity, which led to spine loss73. Moreover, increased RHOA levels cause reductions in dendritic spine several additional studies have shown that although density71,72, whereas RHOA inhibition or knockdown of RHOA activation or loss of a RHOA inhibitory cas- the RHO activator guanine nucleotide exchange factor 1 cade in mature neurons leads to a dramatic shrinkage (GEF1) increases spine density71,73. Inhibition of the major of dendrite arbors, it does not affect dendritic spine RHO target RHO-associated protein kinase (ROCK) can stability68,76,77. Overall, it appears that specific activity block spine loss resulting from increased RHOA protein patterns, developmental timing or other biological factors levels72. These studies suggest that RHO–ROCK signalling greatly influence the impact of RHOA–ROCK signal- antagonizes the stability of previously formed dendritic ling on dendritic spine stability. Ongoing work in the spines, although the target of RHO signalling in regulating field should identify how these factors influence RHOA– spine stability is not clear. RHO signalling through ROCK ROCK activation and its effect on spine stability.

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ABPs have key roles in long-term spine stabilization. these new spines lack PSD95‑positive PSDs, indicating Dendritic spines are enriched in several ABPs that medi- that they are unable to form functional synapses96,97. This ate activity-dependent effects on dendritic spine shape, increased spine lability and defective synapse formation size and stability, including cortactin, drebrin, ABP1, probably contributes to the observed reductions in spine caldesmon, myristoylated alanine-rich C‑kinase sub- density in adult Add2−/− mice97. Together, these studies strate (MARCKS) and β‑adducin78–82. Cortactin is an illustrate how the organization of spinoskeletal structure F‑actin-binding protein that stimulates new actin branch and spine stability, and the influence of synaptic activity nucleation through the ARP2/3 complex and stabilizes on this stability, is coordinated by multiple ABPs. the association of the new branch with the mother fila- ment83. This protein is enriched in the spinoskeleton core, EPHB signalling through FAK stabilizes mature den‑ where about 50% is stable (as measured by fluorescence dritic spines. Although originally identified as a kinase recovery after photobleaching; see FIG. 3)45,84. Cortactin associated with integrin signalling, FAK can be acti- has also been reported to interact with several proteins vated by a large variety of cell surface receptors that with known or suspected roles in long-term spine stabi- regulate spine stability, including EPHB receptors (FIG. 3). lization, including cortactin-binding protein 2, the post- Disruption of EPHB2 signalling or knockdown of FAK synaptic density (PSD) scaffolding protein SHANK3, in established hippocampal neuron cultures leads to a p130 Cas-associated protein (p140CAP; also known as significant loss of mature mushroom-shaped dendritic SRCIN1) and tyrosine protein kinase ARG (also known spines and an increase in immature thin ‘filopodia-like’ as ABL2) (see below)76,85–88. spines98. FAK can activate RHOA through 190 kDa GEF Because of its interactions with SHANK3 and F‑actin (p190RHOGEF; also known as ARHGEF28), and sub- within spines, it is perhaps not surprising that cortactin sequent downstream inactivation of the actin-severing localization and function in spine stability are tightly reg- protein cofilin via phosphorylation by LIM kinase may ulated by synaptic activity. Indeed, chronic stimulation mediate spine stabilization. Consistent with this model, of cultured hippocampal neurons by bath application of an inactive phosphomimetic cofilin mutant can block NMDA, which is known to destabilize dendritic spines, the spine destabilization induced by loss of EPHB2–FAK leads to cortactin redistribution away from postsynaptic signalling98. In addition, activated FAK also recruits and sites to dendritic shafts76,84,89,90. Interestingly, activation activates SRC family kinases to phosphorylate p130 Crk- of the receptor tyrosine kinase TRKB (also known as associated substrate (p130CAS; also known as BCAR1), NTRK2) by its ligand brain-derived neurotrophic factor a scaffolding molecule that promotes RAC1 activation99. (BDNF) leads to the opposite effect, increasing cortac- Interestingly, knockout of the SRC family kinase FYN tin localization to spines. These findings indicate that leads to a progressive age-dependent loss of dendritic TRKB and the NMDA receptor (NMDAR) exert oppo- spines in mice100, whereas knockdown of p130CAS leads site effects on spine stability via their different effects to spine loss in established hippocampal neuron cul- on cortactin levels in spines. Together, these findings tures101. These manipulations suggest a mechanism by reveal cortactin as a key effector of spine stability which spine shrinkage and/or destabilization results from downstream of both stabilizing and destabilizing cues. disruption of FAK-mediated RAC1 activation. Drebrin, MARCKS and ABP1 also have key roles in dendritic spine morphogenesis, particularly in the BDNF signalling through TRKB influences spine size and transition from thin immature dendritic spines to stability. BDNF is released from both pre- and postsyn- larger, more stable mushroom spines79,91,92. Knockdown aptic compartments during neuronal depolarization or of MARCKS in mature hippocampal neuron cultures direct stimulation with glutamate, where it has crucial reduces F‑actin content in spines, leading to fewer spines roles in synapse development and stability102–106. During that are smaller in size78. Phosphorylation of MARCKS development, BDNF signalling through presynaptic by protein kinase C (PKC) inhibits its association with TRKB receptors has been shown to stabilize neurotrans- the membrane and its ability to crosslink F‑actin, and has mitter release sites and reduce synapse turnover105,106. been implicated in activity-dependent control of spine Interestingly, blockage of NMDARs also disrupts clus- size and stability93–95. Indeed, direct activation of PKC tering of the presynaptic release sites, but this can be in mature hippocampal neuron cultures causes spine rescued by application of BDNF, suggesting that NMDA- shrinkage and loss. Importantly, PKC-induced spine mediated BDNF release is critical for stabilization of the destabilization can be blocked using a non-phosphoryl- presynaptic compartment105. Although the mechanism atable MARCKS mutant78, indicating that PKC triggers by which BDNF–TRKB signalling regulates presynaptic spine destabilization through inhibition of MARCKS. stabilization is not completely clear, it is possible that β-adducin proteins cap actin filaments, and removal TRKB signalling to cortactin or other ABPs leads to sta- of these caps allows the polymerization process to be bilization of the presynaptic actin network, which in turn coordinated by inputs from different signalling pathways stabilizes the synaptic vesicle release machinery. into changes in actin structure. Dendritic spines of mice In the postsynaptic neuron, many actin regulatory lacking the gene that encodes β‑adducin (Add2−/− mice) pathways central to spine enlargement and stabilization are relatively normal in appearance but exhibit greatly are modulated by BDNF–TRKB signalling. BDNF stim- increased rates of turnover96. Increasing sensory stimula- ulates activity of serine/threonine-protein kinase PAK (a tion by housing these mice in enriched environments can major target of RAC) and also induces increased inhibi- induce new spine formation, but a significant fraction of tory phosphorylation of cofilin, which is associated with

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increased spine size and stability (see above). TRKB can which promotes spine stability. Indeed, fusion of just the also activate the GTPase RAS, which promotes spine ARG F‑actin-binding domains to cortactin results in its enlargement and increased stability107. Interestingly, accumulation in spines and rescues spine instability in BDNF signalling through the TRKB receptor can inter- ARG knockdown neurons76. Interestingly, the synapses act with activity-induced molecular pathways to stabilize that remain on ARG knockdown neurons are enlarged dendritic spines. For example, exposure of hippocampal and exhibit increased mini-excitatory postsynaptic cur- slices to modest electrophysiological stimulation and rent amplitudes, consistent with increased signalling by BDNF leads to greater increases in spine F‑actin con- postsynaptic AMPA receptors and NMDARs. NMDAR tent than either treatment alone, suggesting a synergistic antagonists prevent cortactin depletion from spines and effect. Furthermore, high-frequency stimulation of pre- rescue spine loss resulting from ARG knockdown in synaptic afferents in hippocampal slices also increases cultured neurons. These observations suggest that ARG spine F‑actin content, which is attenuated by blocking may also attenuate NMDAR activity to counter its antag- TRKB signalling108. onism of cortactin localization to spines. One possibility is that ARG may regulate NMDAR subunit phospho- Integrin β1 signalling through ARG and cortactin medi‑ rylation127,128, which is known to regulate localization and ates dendritic spine stability. Integrins are heterodimeric trafficking of NMDARs129. Thus, ARG appears to serve receptors that are composed of α and β subunits and are as a general brake on the spine destabilizing influence of widely expressed in the nervous system, with numerous excessive NMDAR signalling. subunits expressed throughout the brain109. They localize to synapses, where they link pre- and postsynaptic mem- Control of dendrite arbor stabilization branes to the extracellular matrix, and are involved in The size and shape of a dendrite arbor determine where signalling complexes that regulate the cytoskeletal struc- it can receive inputs and how it can integrate into neu- ture. The integrin β1 subunit is localized particularly to ral networks. Dendrite branches are especially stable the centre of the PSD110. Importantly, integrins appear to in mature neurons, and this stability is essential for the be involved in the stabilization of LTP, which is associ- durability of neural networks. Dendrite branches are sup- ated with a rapid, long-lasting increase in polymerized ported and maintained through the combined actions of F-actin and an increase in spine stability111–113. Indeed, an extensive network of microtubules and associated inhibition or loss of integrin α3β1 destabilizes spines and proteins. reduces synapse density during adolescence and early adulthood (postnatal day 21 (P21)–P42) in mice101,114,115. Role of microtubule-based trafficking in stabilizing den‑ The extracellular molecules that activate integrins at syn- drite arbors. Drosophila melanogaster dendrite-arboriz- apses are not known but may include both traditional ing sensory neurons undergo complete dendrite arbor extracellular matrix molecules, such as laminins and loss during metamorphosis, and this model system has thrombospondins, and guidance cues, such as netrins been particularly useful for studying the mechanisms that and reelin, which can interact with integrins116–119. control microtubule networks in dendrite branch stabil- Integrins signal through the ABL family kinases, ity. Severing of individual dendritic branches is initiated ABL and ARG in vertebrates, to coordinate changes in by a localized depletion of microtubules and thinning of cytoskeletal structure120. Importantly, biochemical and the dendrite branch, followed by severing and fragmen- genetic experiments indicate that the integrin β1 cyto- tation of the branch130. Genetic studies indicate that the plasmic tail binds directly to ARG and that this interac- microtubule-severing protein Katanin 60 is crucial for tion is crucial for dendritic spine and dendrite stability this process131. These observations underscore the cen- (FIG. 3). ARG is particularly abundant in the nervous tral role of microtubule integrity for maintaining overall system121, where it localizes to dendritic spines76,114,122, dendrite arbor stability. and Arg−/− mice exhibit widespread losses of spine and Dendrites are composed of large amounts of pro- synapse density throughout the cortex and hippocampus teins and lipids, which must be continually replenished. (20–35% depending on the brain region) as they mature Although this is partially accomplished by local pro- to adulthood77,123. Interestingly, these phenotypes closely tein synthesis, other components must be synthesized resemble those observed in conditional knockout mice and processed elsewhere and trafficked to dendrites. that have neurons lacking the integrin subunit α3 or β1 Microtubules play crucial parts in the protein and lipid (REFS 101,114,115), which is consistent with a role for trafficking events that are required to sustain dendrite ARG as a major downstream mediator of spine stabili- structure, and perturbations that disrupt trafficking of zation by integrin α3β1. microtubule-based cargoes have devastating effects on ARG acts by binding cooperatively to actin filaments dendrite formation during development and stability. and preventing their depolymerization124,125. ARG bind- For example, disruptions in microtubule orientation and ing to actin filaments also changes their helical pitch126, spacing correlate with disruptions of local dendrite archi- promoting increased cortactin binding, which may fur- tecture and have been noted in neurons from individuals Golgi outposts ther stabilize filaments125. Knockdown of ARG in cul- with neurodevelopmental disorders132,133. Moreover, the Clusters of Golgi-like cisternae tured hippocampal neurons leads to cortactin loss from Golgi apparatus in the soma is highly polarized towards that reside in neuronal Golgi outposts processes and act as local spines, a parallel decrease in spine F‑actin content and a dendrites, and , which serve as local sites 76 trafficking centres for 50% loss of spines . These data suggest that binding of for post-Golgi trafficking, and recycling of membrane- membrane-bound vesicles. an ARG–cortactin complex to F‑actin stabilizes actin, based cargoes as well as new microtubule nucleation,

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are localized at many dendrite arbor branch points134,135 membrane-associated p120RASGAP, which allows (FIG. 2). Mutations in the microtubule-based motors p190RHOGAP to inhibit membrane-bound active dynein and kinesin also significantly disrupt traffick- RHO145,146. p190RHOGAP phosphorylation and ing within the dendrites as well as proper localization p190RHOGAP–p120RASGAP complex formation of Golgi outposts, and lead to deficits in dendrite arbor are decreased in the brains of Arg−/− mice , leading to formation and stabilization136,137. Nuclear Dbf2‑related net increases in RHO activity. Importantly, dominant- kinase 1 (NDR1; also known as STK38) and NDR2 (also negative RHOA or reducing the gene-dosage of Rock2 known as STK38L), which are key regulators of dendrite suppresses the dendrite branch loss observed in Arg−/− arbor development in vertebrates, have recently been mice76,77. Together, these studies indicate that the integ- shown to target several proteins with known or suspected rin α3β1–ARG–p190RHOGAP axis preserves dendrite roles in protein trafficking138. structure in early adulthood by attenuating RHO activity. Identification and characterization of the integrin ligand Activity and BDNF signalling through the TRKB recep‑ that activates this pathway should reveal how and why tor regulate microtubule-binding proteins to stabilize this pathway is activated to stabilize branches and spines. dendrites. Neuronal activity is a potent stimulator of dendritic remodelling and stabilization in developing Pathological dendritic destabilization by RHO–ROCK neurons, and the neurotrophic factor BDNF, signalling signalling. Although most dendrite arbors exhibit long- through TRKB, appears to be an important mediator of term stability, there are circumstances under which this effect60,61,139 (FIG. 4). Experimental approaches such dendrite stability is compromised, such as in neurode- as membrane depolarization, repetitive electrophysi- generative diseases and after an excitotoxic insult during ological stimulation and direct application of NMDA stroke (see below). The GTPase RHO appears to be a cen- have all been shown to promote BDNF release at syn- tral mediator of this dendrite destabilization. For exam- apses in cultured hippocampal neurons102–104. Moreover, ple, expression of constitutively active RHO, or reduced brain-specific ablation of BDNF or its receptor TRKB activation of RHO inhibitors, such as p190RHOGAP leads to a striking shrinkage of cortical excitatory neu- (see above), destabilizes growing147–150 and mature68,151 ron dendrites from early adolescence to adulthood140,141. dendrite arbors (FIG. 4). RHO-mediated activation of The molecular mechanisms by which synaptic activity ROCK probably acts through several mechanisms to stimulates neurotrophic factor signalling that leads to disrupt dendrite architecture. Inhibition of ROCK can dendrite microtubule stability are coming into focus. promote microtubule assembly and the stabilization Direct application of BDNF or nerve growth factor to of dendrite-like neuronal processes in cells, suggesting cultured neurons is sufficient to induce expression of that ROCK antagonizes dendrite stability by disrupting the microtubule-stabilizing proteins MAP1A and MAP2 microtubules152. Indeed, ROCK phosphorylates MAP2 (REFS 58–61). In addition, BDNF also induces expression on a site in its microtubule-binding region. ROCK- of cypin, a guanine deaminase that promotes micro- mediated phosphorylation of a similar conserved site in tubule assembly, and dendrite formation and stability. tau disrupts its interactions with microtubules153. MAP2 These data suggest that BDNF signalling through TRKB phosphorylation in other contexts has been shown to converges on microtubule-binding proteins to mediate reduce its ability to bind microtubules and promote their dendrite stability142,143. assembly154,155. Activated RHO also suppresses translation of the microtubule stabilizer cypin (see above), but Integrin signalling to p190RHOGAP attenuates RHO cypin overexpression in cultured hippocampal neurons activity to stabilize dendrite branches. In addition to can overcome the reduction in dendrite arbors induced regulating dendritic spine stability, as discussed above, by increased RHO activity151. These studies indicate that integrin α3β1 signalling through the ABL and ARG the loss of MAP- and cypin-mediated microtubule sta- kinases is crucial for dendrite branch stabilization (FIG. 4). bilization contributes to RHO-mediated dendrite arbor Dendrite arbors of cortical or hippocampal excitatory destabilization. neurons lacking either ARG or both kinases develop normally until P21, approaching their fully mature size. Coordination of spine and dendrite stability As the mice reach adulthood, however, dendrite arbor Although the mechanisms that regulate dendritic spine size reduces significantly, leading to an overall thinning and dendrite branch stability have been discussed sepa- or shrinkage of the cortex and hippocampus77,144. Similar rately so far, there are several instances in which signal- phenotypes are observed in hippocampal CA1 neurons ling between the compartments influences stability. This deficient in the integrin subunit α3 or β1, both of which crosstalk is particularly evident under conditions in which are key upstream regulators of ABL and ARG. However, activity-driven circuit remodelling requires a coordinated no defects in dendrite branch stability are observed in shaping and stabilizing of spine and dendrite structure. animals with disruption of integrin subunit α5 or β3, suggesting that integrin α3β1 interacts selectively with Activity-driven spine enlargement and stabilization. ARG to control dendrite stability. Activity and activity-mediated neurotrophic factor sig- The 190 kD RHO GTPase-activating protein nalling affects dendritic spine size and stability through- (GAP) A (p190RHOGAP) is a major ARG substrate out a neuron’s lifetime, including mature cortical and in the postnatal brain145. ARG-mediated phospho- hippocampal neurons. Growing microtubule ends rylation of p190RHOGAP promotes its binding to are enriched in ‘plus end tip-binding proteins’, notably

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ARP2/3 complex MAP1A Cortactin Microtubule Coﬁlin Microtubule monomers Cypin Vesicle F-actin a Microtubule-based BDNF AMPAR MAP2 motors PSD95 TRKB

Cortactin +TIPs b stabilizes actin Microtubule-based cargoes Nucleus

MAP1A MAP2 P P P P Cypin mRNA ROCK P Cypin F-actin translation destabilized RHO GDP Ribosome RHO RHO p190RHOGAP GTP GTP MEK p120RASGAP ARG Ca2+ c Integrin α3β1 Extracellular NMDAR matrix

d Glutamate-containing vesicle

Figure 4 | Pathways that mediate crosstalk between dendritic spines and dendrite arbors.Nature Several Reviews pathways | Neuroscience mediate crosstalk between the spine and shaft cytoskeletons. a | Brain-derived neurotrophic factor (BDNF) signalling through the receptor tyrosine kinase TRKB triggers assembly of microtubules containing plus end tip-binding proteins (+TIPs), such as end-binding protein 3 (EB3), to target to spines and allow delivery of cargoes that stabilize spine structure. BDNF also promotes accumulation of cortactin in spines to stabilize F‑actin structure. b | BDNF triggers increased expression of microtubule-associated protein 1A (MAP1A), MAP2 and cypin, all of which promote microtubule stabilization in the dendrite shaft. c | Integrin α3β1 signalling through the receptor tyrosine kinase ARG and 190 kD RHO GTPase-activating protein A (p190RHOGAP) attenuates RHO activity to preserve dendrite shaft structure. d | Increased AMPA recptor (AMPAR) and NMDA receptor (NMDAR) activity, such as that associated with stroke, results in Ca2+ influx that activates RHO–RHO-associated protein kinase (ROCK) signalling which phosphorylates MAPs and dissociates them from microtubules. Increased levels of ROCK also inhibit localized cypin translation, which further destabilizes spines. NMDAR hyperactivation also triggers cortactin relocalization to the dendrite shaft, which destabilizes dendritic spine F‑actin. MEK, MAPK/ERK kinase.

end-binding protein 3 (EB3; also known as MAPRE3). that are generally associated with greater stability. By con- Live-imaging studies involving tracking of the growing trast, disruption of microtubule dynamics using drugs microtubule tips using EB3‑fluorescent protein fusions or by knocking down EB3 leads to reductions of PSD95 have found that neuronal activity results in EB3‑labelled and p140CAP in spines, destabilizing some of them and microtubule tips making transient visits to dendritic decreasing the size of those remaining85,156,158. spines85,156–158 (FIG. 4). Microtubule invasion of individual Intriguingly, these microtubule ‘visits’ to dendritic spines is accompanied by increased accumulation of spines are greatly increased by membrane depolariza- PSD95 and the EB3‑binding protein p140CAP, both of tion or stimulation with BDNF85,156–158 (FIG. 4). A decrease which are required for spine maintenance85.157. These in BDNF-induced structural reinforcement of the spine events are accompanied by increased spine F‑actin con- may explain the reductions in hippocampal spine density tent and spine enlargement, both of which are features reported in mice with reduced gene-dosage of the BDNF

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receptor Trkb 159. Conversely, treatment of cultured neu- in synaptic inputs, by experimental deafferentation or rons with a chemical long-term depression induction pro- removal of sensory inputs, leads to dendrite loss in a tocol arrests the mobility and targeting of EB3‑positive wide variety of model systems7–9,77,161. Both Ca2+/calmo- microtubule tips, preventing them from entering the dulin-dependent kinase II (CaMKII) and MEK (MAPK/ spine160. Under these conditions, EB3 relocates to the den- ERK kinase; also known as MAP2K) signalling mod- dritic shaft, where it associates with microtubule-bound ules are crucial for activity-dependent dendrite stabili- MAP2, and the loss of microtubule targeting to the spines zation in cultured sympathetic neurons60. Interestingly, leads to spine shrinkage and loss. arrest of dynamic dendrite behaviour and premature stabilization of dendrite arbors can also be achieved Stabilization of dendrites by synaptic activity. As men- via expression of a constitutively activated form of the tioned above, new synapse formation is essential for postsynaptic signalling protein CaMKII139. CaMKIIα has dendrite stabilization in developing neurons, but main- many substrates that can affect the actin cytoskeleton tenance of this synaptic input appears to be crucial for within the spine, including the RAC activators T lym- ongoing dendritic stability18,19, Accordingly, a reduction phoma invasion and metastasis-inducing protein 1 (REF. 162) and kalirin 7 (REF. 163), and the RAS GTPase activating protein SYNGAP164. Although CaMKIIα and ADDL its substrates are predominantly localized to dendritic C FYN Axon PrP NMDAR spines165, it can relocalize to dendritic shafts upon stimu- 166 P lation in cultured neurons . The exact mechanism by Stabilizing Kinases tau molecules which CaMKIIα functions to stabilize dendrites awaits a P P further study. P P b β4 tubulin Events that destabilize spines and branches Tau aggregates Several human brain diseases that arise later in life are P P c associated with losses of dendritic spine density and den- P P P P drite arbor complexity, including psychiatric diseases, P P such as schizophrenia and depression, and neurode- GATA1 generative diseases, such as Alzheimer’s disease167. The Destabilized microtubules consequent loss of connectivity at both the local and inter- regional level is thought to contribute to disease symp- Dendrite TRKB Synapse protein toms. Emerging evidence suggests that the pathology associated with these diseases specifically targets dendritic F-actin Stress? spine and dendrite branch stabilization mechanisms. Loss of loss BDNF–TRKB Corticosteroids? d Roles of ADDLs and tau in spine and dendrite instabil‑ ity in Alzheimer’s disease. Extensive synapse loss and Glutamate- TRKB e containing vesicle dendrite arbor atrophy is observed in Alzheimer’s dis- ease168–171, and the reductions in synapse number strongly correlate with the magnitude of cognitive and memory impairment172,173. Work over the past several years has focused on soluble amyloid-β (Aβ)‑derived diffusible ligands (ADDLs; BDNF also known as Aβ oligomers) as key triggers for spine loss and dendritic atrophy in Alzheimer’s disease pathol- Cortactin MAP1A Microtubule Nucleus ogy174 (FIG. 5). Application of ADDLs to cultured neurons or slices has been found to lead to a significant loss BDNF MAP2 Tau of dendritic spines175–177. Insight into the mechanism underlying this effect has been gained from the find- Figure 5 | Disruption of spine and dendrite stabilization mechanisms in ing that binding of ADDLs to the cellular form of the disease. Disease pathology specifically disrupts dendritic spine and dendrite arbor prion protein (PrPC) triggers FYN-mediated phospho- Nature Reviews | Neuroscience stabilization. a | In Alzheimer’s disease, amyloid-β-derived diffusible ligands (ADDLs) rylation of the GluN2B (also known as NR2B) subunit C bind to the cellular prion protein (PrP ) receptor and activate receptor tyrosine kinase of the NMDAR178. This initially leads to an increase in FYN as well as other kinases. This triggers transient NMDA receptor (NMDAR) the number of NMDARs at the postsynaptic surface phosphorylation, followed by NMDAR internalization and tau hyperphosphorylation followed by a persistent loss of surface NMDAR by and accumulation in spines and shafts. b | There is also redistribution of tau from axons 175,176,178 (FIG. 5) to dendrites. c | In major depressive disorder (MDD), upregulation of GATA-binding endocytosis . The initial rise in NMDAR factor 1 (GATA1) inhibits expression of the dendrite shaft component β4 tubulin and surface activity may contribute to spine destabilization, also several genes required for synaptic function. d | Stress and increased corticosteroid possibly by triggering the loss of drebrin and cortactin levels, which are known risk factors for MDD, inhibit expression of brain-derived from the spine175. However, NMDAR activation probably neurotrophic factor (BDNF) in the presynaptic neuron and its receptor the receptor also mediates spine destabilization by signalling through tyrosine kinase TRKB in the postsynaptic neuron. e | This disrupts spine and dendrite calcineurin179 to activate cofilin180, which as mentioned stabilization by this BDNF–TRKB axis. AMPAR, AMPA receptor. above, regulates actin disassembly. Both calcineurin

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and cofilin have been found to be required for ADDL- Disruption of spine and dendrite stability in stroke. induced spine destabilization176. ADDL treatment also Ischaemic events such as stroke cause destabilization of disrupts the surface localization of other receptors, such spines and dendrites on neurons within and adjacent to as EPHB2 (discussed above), that are also important the infarct area. This destabilization may clear dendrites regulators of spine stability175. away from areas in which damage or death of afferents ADDLs also trigger significant dendrite arbor shrink- has disrupted connectivity, thus freeing up molecular age179. ADDLs stimulate the activities of several kinases components and metabolic resources to remodel the (including MARK1, p70S6K, BRSK and FYN) that can dendrite arbor during the recovery phase. In a murine phosphorylate the microtubule-binding protein tau181,182. ischaemia model, the reductions in dendrite arbor size Hyperphosphorylated tau relocalizes from a primarily in the peri-infarct region that follow stroke are closely axonal distribution to the soma and some dendrites matched by growth of distal dendrites during the recov- (FIG. 5), an effect that is associated with a significant ery period196, suggesting that the neuron may attempt to reduction in microtubules and localized dystrophic reach some homeostatic ‘set point’ in dendrite size and beading of the dendrites179,181. Interestingly, these tau synaptic inputs as it reintegrates into networks. missorting events are also accompanied by reductions Increased NMDAR activity and the resulting excito- in spine density181. Although a small portion of tau is toxicity has long been known to mediate neuronal dam- normally found in spines, where it tethers FYN to the age and death after stroke (for a review, see REF. 197). PSD183, hyperphosphorylated tau accumulates in spines, Application of NMDA to cultured neurons or focal where it interferes with NMDA localization to the spine application of NMDA to specific brain areas in vivo and synaptic anchoring, which contributes to cytoskeletal has been widely used to model this glutamate-induced destabilization183,184. Thus, ADDL-induced tau phospho- excitotoxicity. rylation contributes to destabilization of both dendrite Whereas prolonged, excessive stimulation leads to arbors and dendritic spines, albeit through distinct neuronal death, excitotoxic but sublethal NMDAR stim- mechanisms. ulation of neurons, either in culture or focally in vivo, causes spine loss, dendrite branch swelling and dendrite Dendritic spine destabilization and dendrite atrophy in shrinkage84,89,198–202. These changes are closely associated depression. MDD is associated with a reduction in syn- with disruptions in actin and microtubule structure apse density185 and reduced dendrite arbor size in the in spines and dendrites, respectively89,127,203. In spines, prefrontal cortex and hippocampus186–189. A recent study the cytoskeletal destabilization appears to be linked found that levels of GATA-binding factor 1 (GATA1), to disrupted functioning of actin regulatory proteins. a transcriptional repressor and master regulator of sev- Accordingly, bath application of NMDA to cultured eral genes that has crucial roles in dendritic spine and hippocampal neurons causes cortactin to relocalize dendrite arbor maintenance, are reduced in the brains from dendritic spines to the dendritic shaft, and this of individuals with MDD185 (FIG. 5). Several GATA1 tar- correlates with the loss of spine stability84,89. In addi- get genes are significantly downregulated in the MDD tion, inhibitors of the cathepsin B family of proteases brain, including β4 tubulin (the major β-tubulin subu- attenuate NMDA-induced spine loss, Indeed, NMDAR nit in neurons) and the membrane trafficking regulators stimulation leads to reduced levels of the spine stabi- RAB3A and RAB4B185. These reductions may cause a lizing protein MARCKS (see above), but this decrease decrease in the size and stability of the microtubule net- can be prevented by cathepsin B inhibition, and this work and perturb the trafficking of key dendritic build- suggests that the spine collapse induced by NMDAR ing blocks that support dendrite structure. This model stimulation results from proteolysis by cathepsin B of is supported by the observation that GATA1 overexpres- MARCKS and possibly other key actin regulatory pro- sion alone is sufficient to significantly reduce dendrite teins203. Increased NMDAR stimulation also disrupts arbor size in cortical neurons185. the targeting of dynamic microtubules to spines, which Chronic stress and the associated increase in circu- prevents delivery of key proteins, including PSD95 and lating corticosteroid levels are major risk factors for the p140CAP160, which are required for spine stability. development of MDD. The application of restraint stress Exactly how increased NMDAR stimulation triggers and the administration of exogenous corticosterone have dendrite branch destabilization is less clear. One poten- both been used as models for MDD states in rodents. tial candidate for mediating these effects is the GTPase Both treatments reduce BDNF and TRKB levels in the RHOA. As noted above, focal synaptic stimulation can hippocampus and prefrontal cortex190–194 (FIG. 5), which activate RHOA in the spine, which can then diffuse into probably explains the reduction in BDNF in the brains of the dendritic shaft67. The summed signalling from a group patients with MDD195. Reduced BDNF signalling would of spines in aggregate, such as that following excessive be expected to compromise the BDNF-mediated sup- NMDAR stimulation, could trigger RHOA–ROCK sig- port of dendritic spine and dendrite arbor stability via nalling at levels sufficient to disrupt MAP function154,155 mechanisms detailed above, thereby contributing to the and cypin expression151, thereby destabilizing the dendritic neuronal destabilization found in this disease. Recent cytoskeleton. In support of this, the ROCK inhibitor fas- work shows that chronic corticosterone treatment also udil can reduce the severity of NMDA-induced damage decreases expression of caldesmon, an F‑actin-stabilizing to the dendrite-rich inner plexiform layer of the retina202. molecule, increasing F‑actin turnover in dendritic spines Moreover, RHO activation and NMDA stimulation have and resulting in smaller, more unstable spines82. both been shown to decrease cypin levels in cultured

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neurons200. Finally, it is possible that NMDA-mediated continue to identify mutations and polymorphisms in disruption of microtubule structure affects dendrites by genes associated with dendrite instability. Understanding disrupting cargo delivery to the dendrite. how these genes contribute to stabilization and how their functions are altered in susceptible individuals should Conclusions reveal new methods to identify those at risk of disease Neurons are continually in a balance between stabiliza- and also suggest preventive or therapeutic approaches for tion and destabilization. In the normal healthy brain, these diseases. this balance is predominantly tipped in favour of sta- Fortunately, our understanding of the biochemical and bilization, but the loss of this balance towards exces- cellular mechanisms that shape and maintain dendrite sive and unregulated destabilization is a major factor in structure is accelerating, as methods to control neuronal various brain disorders. The identification of pathways activity and fluorescent probes for measuring biochemical that mediate stabilization of spines and dendrites and events in neurons become even more widely used. The the evidence that they are disrupted in brain diseases goal of these studies should be to both reveal the timing raise their profiles as possible targets for therapeutic and distribution of key signalling events and causally intervention. However, our understanding of these relate them to changes in dendritic spine and dendrite mechanisms, which are vital for our brain function over structure and function. Along the way, we should learn several decades, is far from complete. why different diseases are associated with different pat- In the future, one challenge will be to understand terns of dendritic spine and dendrite destabilization. It is how different activity patterns influence these stabiliza- possible that in diseases in which multiple pathways are tion mechanisms at the biochemical and physiological disrupted (for example, Alzheimer’s disease), ‘combina- level and whether these can be manipulated to increase tion therapies’, such as those increasingly used to treat stability. Genome-wide searches for genes associated cancer, may prove especially efficacious to treat some with psychiatric and neurodegenerative disorders will diseases.

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